How can trustworthy scholars differ in classifying a Hadith as either "Hasan" (good) or "Da'if" (weak), when the the Sunnah is preserved just like the Qur'an?

The science of Hadith, like other sciences, is such that whoever is qualified to examine it and finds a view to be preponderant, let him state it. And whoever is not qualified for that, let him follow someone he trusts, and by that, his responsibility is absolved. The disagreement among scholars regarding the authentication of a Hadith does not mean that revelation is not preserved. Some texts are definitively proven (qat'i al-thubut), and some are speculatively proven (dhanni al-thubut). Disagreement in authentication is one of the greatest causes of difference among scholars, and it has profound wisdoms, including increasing the rewards of the diligent researchers (mujtahidin) and providing ease for the common people.